Master of Science Thesis in Electrical Engineering Department of Electrical Engineering, Linköping University, 2020 Alternative Input Devices for Steer-by-Wire Systems Casper Christiansen and Viktor Alkelin Master of Science Thesis in Electrical Engineering Alternative Input Devices for Steer-by-Wire Systems Casper Christiansen and Viktor Alkelin LiTH-ISY-EX--20/5296--SE Supervisor: Victor Fors isy, Linköpings universitet Matthijs Klomp Solution Architect, Volvo Cars Examiner: Jan Åslund isy, Linköpings universitet Division of Vehicular Systems Department of Electrical Engineering Linköping University SE-581 83 Linköping, Sweden Copyright © 2020 Casper Christiansen and Viktor Alkelin Abstract With the recent push towards autonomous cars, a traditional steering wheel with its mechanical connection between the road and driver may soon be unnecessary. To facilitate interior design and lower production costs whilst still maintaining a manual alternative for maneuvering, an alternative steering input device relying on Steer-by-Wire technology is investigated. In order to finish the investigation and development of the steering device within the time-span of a master thesis, the limitation to only investigate the design of a hand wheel was established. The finished alternative steering device utilises an optical encoder for position measurement and a brushless direct current (DC) motor with a planetary gearbox for force feedback. Open-loop speed control proved to be insufficient with the available hardware. Instead, an approach of two PD-controllers regulating the angular error between the steering rack and the steering device was implemented successfully. Initially, mathematical models of the system components were derived and im- plemented in Mathworks Simulink. The transition from models to test rig im- plementation proved to be difficult due to unknown parameters in the hardware components such as embedded controllers in the steering gear and the internal works of the sensor emulator used to control the steering gear. By modifying pa- rameters in accordance with system identification measurements performed on the test rig, the models could be validated. At the end of the project, a Volvo S60 was made available and the steering de- vice was tested with real world driving. It was discovered that controllers tuned only for good reference following in the test rig did not translate to good drive- ability as the controller allowed for overly aggressive maneuvers. Following some in vehicle tuning, the proposed solution performed well during testing with sur- prisingly high drive-ability. For future iterations of similar hand wheel design projects, a user study was per- formed with regards to user experience, hand wheel size and perceived drive- ability. iii Acknowledgments We would like to extend our gratitude to Volvo Cars for such an exciting oppor- tunity to test and prove our engineering knowledge. With special thanks to Matthijs Klomp for his personal interest and guidance in our thesis project, Georgios Minos for the administrative aid and we also want to acknowledge everyone at steering, who helped us answering questions and supplying data. From Linköpings University we want to thank Victor Fors for all his work improv- ing the quality of our master thesis and Jan Åslund for assuring the academical reach. Not to be forgotten is Adrian Aune, William Andersson, Axel Jyrkäs, Gustav Ljungquist and Harish Kumar for the good company during our lunches. Finally, we do not in any way want to thank COVID-19 for all its complications to our thesis work and potential future careers. Stay safe Gothenburg, June 2020 Casper Christiansen and Viktor Alkelin v Contents List of Figures ix Notation xi 1 Introduction 1 1.1 Introduction . 1 1.2 Problem description . 2 1.3 Approach . 2 1.4 Delimitations . 3 1.5 Related research . 4 1.5.1 Objective metrics and test scenarios for steering systems . 5 1.5.2 Previous work . 5 1.6 Outline . 6 2 System Description 7 2.1 Steer-by-Wire . 7 2.1.1 Electronic control unit (ECU) . 8 2.1.2 Controller area network (CAN) . 9 2.2 Force feedback in Steer-by-Wire systems . 9 2.2.1 Brushless DC motor . 10 2.3 Sensors . 10 2.3.1 Angle sensor . 10 2.3.2 Torque sensor . 11 3 System control strategies 13 3.1 Open-loop speed control . 13 3.2 Closed-loop angle control . 14 3.3 Reference generated feedback . 15 4 Implementation 17 4.1 Hardware implementation . 17 4.1.1 Steering input versions . 18 4.1.2 Force feedback motor . 19 vii viii Contents 4.1.3 Network Interface - Prototype ECU . 20 4.1.4 Steering gear . 22 4.1.5 Sensor emulator . 22 4.2 System safety . 23 4.3 Vehicle implementation . 24 5 System modelling 27 5.1 Steering device and force feedback motor . 27 5.1.1 Force feedback motor and planetary gear . 28 5.1.2 Driver model . 32 5.1.3 Variable steering ratio and feedback . 32 5.2 Steering rack and EPAS motor . 32 6 Results 39 6.1 Viability of controller design . 39 6.2 Closed-loop angle control system - Chirp signal response . 40 6.3 Vehicle test results . 42 6.4 User study . 43 7 Discussion 47 7.1 Discussion and analysis . 47 7.1.1 Model implementation . 48 7.1.2 Vehicle implementation . 48 7.1.3 The implementation of variable ratios and gains . 50 8 Summary 51 8.1 Summary and conclusions . 51 8.2 Future work . 52 Bibliography 55 List of Figures 2.1 Comparison between conventional and SbW systems . 7 2.2 Typical encoder pulse train . 11 3.1 Open-loop controller . 14 3.2 Closed-loop angle controller . 14 3.3 Closed-loop angle controller with reference generator . 15 4.1 Steer-by-wire implementation overview . 17 4.2 Steering device design 1 with small (65 mm) diameter and a re- tractable Brodie knob . 18 4.3 Steering device design 2 with larger diameter (80 mm) and perma- nent Brodie knob . 19 4.4 Steering device design 3 with medium (70 mm) diameter and hooks 19 4.5 Description of the different BLDC control strategies . 20 4.6 Rotary encoder used on the motor shaft . 20 4.7 Operational amplifier circuit with voltage dividers simulated in LTspice . 21 4.8 SPA test rig used during the implementation . 22 4.9 Torque sensor safety flowchart . 23 4.10 Steering device placement in the centre console . 25 4.11 In-car use of device . 25 4.12 VN8911 and sensor emulation box placement on the arm-rest be- tween the rear seats . 25 5.1 Component brakedown of the steering device with force feedback motor and planetary gearbox . 28 5.2 Force feedback system validation for a PWM step of 30% . 30 5.3 Force feedback system validation for a PWM step of 50% . 30 5.4 Force feedback system validation for a PWM step of 80% . 30 5.5 Model validation for ramp signal with slope of 1 from 0 to 1 . 31 5.6 Model validation for ramp signal with slope of 1 from 0 to 2 . 31 5.7 Steering rack and EPAS components . 33 5.8 Simulink sub-system of steering gear . 36 5.9 Step responses and model validation for steps of 0.8 Nm. 37 5.10 Step responses and model validation for steps of 1 Nm. 37 ix x LIST OF FIGURES 6.1 Modelled versus simulated steering device angles . 41 6.2 Modelled versus simulated rack pinion angles . 41 6.3 Angular results from test drive on Volvo test track . 42 6.4 In what scenario do you see yourself driving a car with an alterna- tive steering device? . 43 6.5 What did you think about the size of the steering device? . 44 6.6 To what extent did you feel that a Brodie-knob was necessary? . 44 6.7 What did you think about the driver experience? . 45 Notation Abbreviations Abbreviation Meaning SbW Steer-by-Wire PID Proportional, Integral, Differential (controller) HID Human Interface Devices DC Direct Current AC Alternating Current BLDC Brushless Direct Current motor MIL Model In the Loop SIL Software In the Loop HIL Hardware In the Loop ECU Electronic Control Unit EPAS Electronic Power Assisted Steering HPAS Hydraulic Power Assisted Steering ECU Electric Control Unit PSCU Power Steering Control Unit EMI Electromagnetic Interference SPA Scalable Product Architecture xi 1 Introduction 1.1 Introduction With the current push towards autonomous cars, the need for a large steering wheel with mechanical connection to the road might be slowly diminishing. New innovative solutions that require less space and add more freedom for vehicle in- terior design may therefore be developed as manual back-ups to the autonomous systems. Former Swedish car manufacturer Saab experimented with a proto- type vehicle as a part of the Pan-European project Prometheus which utilized a joystick-type steering device as early as 1992. Although the project never left the research stage, some promising results were found in terms of reported intuitive steering feel after some habituation [1]. Steer-by-Wire (SbW) systems rely on sensors, controllers and motors in order to electronically transmit driver steering input to a motor located on the steering gear assembly in combination with another motor used for providing the driver with road feedback. In comparison, a traditional steering system transmits steer- ing wheel torque mechanically to the steering gear assisted with either electronic power assisted (EPAS) or hydraulic power assisted systems (HPAS). The possible benefits of SbW systems include: • Space-savings and reduced manufacturing costs • Facilitate implementation of driver-assistance systems for improved road safety • Increased vehicle interior design freedom • Variable input/output steering ratios and feedback in different situations 1 2 1 Introduction Still to date, the steering standard in the automotive industry is a mechanical connection between the driver and the wheels, despite a long history of extensive research about SbW systems [2].